1. Field of the Invention
The present invention relates to subscriber telephone instuments of the electronic-microprocessor type, and more particularly to telephone instruments which require fast connect and disconnect functions to the speech network, as a part of such operations as dialing, hold or flash signaling.
2. Background Art
Many previous designs of network disconnect circuitry involve the use of series pass elements such as a PNP type transistor. A prior art example of such a design is found in the "Linear II" telephone manufactured by GTE Communication Systems Corporation. In this particular implementation a Darlington PNP transistor arrangement was used to achieve the required saturation level of the switch. A third NPN type transistor was required to switch the two PNP transitors of the Darlington configuration "on" or "off". Each of the transistors used in this circuit implementation were high voltage types in order to withstand lightning surges which may appear on the telephone subscriber line. The described circuitry however does not protect the speech network from voltage surges. Instead it uses an external metalic oxide varistor for protection. For bias purposes in this arrangement, a pulldown resistor having a value on the order of 24,000 ohms was employed. The resulting low level of DC bias current combined with a high level of AC impedence, permits the electronic switch to become virtually transparent in regard to the receive, sidetone and transmit characteristics of a conventional hybrid voice network circuit. However, the circuit advantages are achieved at the cost of an extra PNP high voltage transistor and an increase in the switches "on" state voltage from approximately 0.3 volts DC to 0.7 volts DC.
An earlier example of the prior art is represented by the "Duofone 160" repretory dial telephone marketed by Radio Shack Incorporated. This circuit uses a single PNP type transistor for a series pass element where the "on" and the "off" state is controlled by a common emitter NPN transistor connected to the PNP transistor's base. AC isolation for these two high voltage transistors is provided by a series connected inductor and resistor coupled between the two transistors. In addition to cost and space factors, this circuit exhibits poor AC isolation at the low end of the voiceband frequencies. It also requires significant DC bias current to maintain the required level of saturation for the PNP transistor switch. Both of these factors can have a significantly degrade the accoustic characteristics of a telephone's voice network.
Accordingly, it is the object of the present invention to provide a low cost circuit which includes a electronic switch for telepone voice network applications which is transparent to the accoustic characteristics of the voice network, exhibits a low "on" state voltage drop and includes a means of surge voltage protection.